GABA-modulin, an endogenous membrane protein, which inhibits non competitively 3H-GABA binding to synaptic plasma membranes, has been isolated and characterized. This protein material is also a potent inhibitor of membrane protein phosphorylation and acts as a competitive inhibitor of the protein kinase substrates. The role of GABA-modulin in the control of GABA receptor system function is presently being investigated.